Recombinant or synthetically produced polypeptides have important applications as pharmaceuticals. Polypeptides such as short peptides, however, often suffer from poor metabolic stability, poor cell penetrability, and promiscuous binding due to conformational flexibility. Various approaches to stabilizing helical peptides have been tried, for example by using intramolecular crosslinkers to maintain the peptide in a desired configuration by introducing disulfide bonds, amide bonds, or carbon-carbon bonds to link amino acid side chains. See, e.g., Jackson et al. (1991), J. Am. Chem. Soc. 113:9391-9392; Phelan et al. (1997), J. Am. Chem. Soc. 119:455-460; Taylor (2002), Biopolymers 66: 49-75; Brunel et al. (2005), Chem. Commun. (20):2552-2554; Hiroshige et al. (1995), J. Am. Chem. Soc. 117: 11590-11591; Blackwell et al. (1998), Angew. Chem. Int. Ed. 37:3281-3284; Schafineister et al. (2000), J. Am. Chem. Soc. 122:5891-5892; Walensky et al. (2004), Science 305:1466-1470; Bernal et al. (2007), J. Am. Chem. Soc. 129:2456-2457; United States Patent Application 2005/0250680, filed Nov. 5, 2004; U.S. Pat. No. 7,192,713 B1 (Verdine et al); U.S. patent application Ser. No. 11/957,325 filed Dec. 14, 2007; U.S. patent application Ser. No. 12/037,041 filed Feb. 25, 2008 and U.S. Pat. No. 5,811,515, the contents of which patents and publications are incorporated herein by reference. There remains a significant need for therapeutic and pharmaceutically useful polypeptides with improved biological properties such as improved in vivo half-lives, efficacy at lower doses or reduced frequency of administration.